Light-emitting diode (LED) devices, also referred to as electroluminescent (EL) devices, have numerous well-known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting. Such devices may be made with light-emitting layers comprising organic materials or inorganic materials such as quantum dots.
Applications of LED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular LED device configuration tailored to these broad fields of applications, all LEDs function on the same general principles. An electroluminescent (EL) medium structure is formed between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the LED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge-carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light-transmissive electrode. If the light-transmissive electrode is between the substrate and the light-emissive elements of the LED device, the device is called a bottom-emitting LED device. Conversely, if the light-transmissive electrode is not between the substrate and the light-emissive elements, the device is referred to as a top-emitting LED device.
The EL medium structure can be formed of a stack of sublayers comprising organic materials that can include small-molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art, for example U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Alternatively, inorganic materials may be employed to form the EL medium structure, for example including core/shell quantum dots formed in a polycrystalline, semiconductor matrix, for example, as taught in pending U.S. application Ser. No. 11/683,479, by Kahen.
Referring to FIG. 6, a top-emitting active-matrix prior-art LED device comprises a substrate 10 having thin-film electronic components 30 formed thereon. A plurality of patterned electrodes 12 separated by electrically insulating materials 32 are covered by a light-emitting layer or layers 14. The light-emitting layers 14 may include charge-control layers such as are known in the art and may, or may not, be patterned with different materials to provide different colored light output over different patterned electrodes 12. Alternatively, the same materials may be employed in the light-emitting layer 14 and a white light emitted. An unpatterned continuous electrode 16 is formed over the light-emitting layers. Upon the provision of a current flow through the light-emitting layer 14 from the electrodes 12, 16 in response to the thin-film electronic elements 30, light is emitted.
LED devices can employ a variety of light-emitting materials patterned over a substrate that emit light of a variety of different frequencies, for example red, green, and blue, to create a full-color display. For small-molecule organic materials, such patterned deposition is done by evaporation and is quite difficult, requiring, for example, expensive metal shadow-masks. Each mask is unique to each pattern and device design. These masks are difficult to fabricate and must be cleaned and replaced frequently. Material deposited on the mask in prior manufacturing cycles may flake off and cause particulate contamination. Moreover, aligning shadow-masks with a substrate is problematic and often damages the materials already deposited on the substrate. Further, the masks are subject to thermal expansion during the organic LED material deposition process, reducing the deposition precision and limiting the resolution and size at which the pattern may be formed.
The use of polymer, rather than metal, masks is known in the prior art. For example, WO2006/111766, published Oct. 26, 2006, by Speakman et al., describes a method of manufacturing comprising applying a mask to substrate; forming a pattern in the mask; processing the substrate according to the pattern; and mechanically removing the mask from the substrate. A method of manufacturing an integrated circuit is also disclosed. However, this method creates significant particulate contamination that can deleteriously affect subsequent processing steps, for example, the deposition of materials or encapsulation of a device. In either of these cases, the subsequent location of a mask over a previously patterned area may damage materials in the previously patterned area or cause particulate contamination in the previously patterned area. Moreover, the removal of a mask may also cause particulate contamination in a previously patterned area.
Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials used for such electrodes include indium tin oxide and very thin layers of metal. However, the current-carrying capacity of electrodes formed from these materials is limited, thereby limiting the amount of light that can be emitted.
In conventional integrated circuits, bus connections are provided over the substrate to provide power to circuitry in the integrated circuit. These busses are located directly on the substrate or on layers deposited on the substrate, for example, on planarization layers. In complex circuits, multiple levels of bus lines are located over the substrate and separated by insulating layers of material. For example, organic LED (OLED) displays sold by the Eastman Kodak Company utilize multiple bus lines located on the substrate and on various planarization layers. However, these busses are not useful to provide power to the light-transmissive upper electrode in an OLED device because conventional photolithographic techniques destroy the organic layers and a typically thin upper electrode employed in a top-emitting OLED device.
U.S. Pat. No. 6,995,035 entitled, “Method of making a top-emitting OLED device having improved power distribution” by Cok and VanSlyke describes a method of making a top-emitting OLED device, includes providing over a substrate, laterally spaced and optically opaque lower electrodes and upper-electrode busses which are electrically insulated from the lower electrodes; depositing an organic EL medium structure over the lower electrodes and the upper-electrode busses; selectively removing the organic EL medium structure over at least portions of the upper-electrode busses to reveal at least upper surfaces of the upper-electrode busses; and depositing a light-transmissive upper-electrode over the organic EL medium structure so that such upper electrode is in electrical contact with at least upper surfaces of the upper-electrode busses. However, such a method tends to form particulate contamination when the organic EL medium structure is selectively removed. The particulate contamination may fall over the EL medium structure and inhibit current flow through the EL medium structure after a subsequent deposition of the light-transmissive upper electrode. This will cause unwanted dark spots.
Commonly assigned U.S. Pat. No. 7,030,553 entitled, “OLED Device having Microcavity Gamut Subpixels and a Within-Gamut Subpixel” by Winters et al, issued Apr. 18, 2006, describes an OLED device including an array of light-emitting pixels, wherein there are at least three gamut sub-pixels that produce at least three gamut-defining sub-pixels and at least one subpixel within the gamut, one of the gamut sub-pixels employing a reflective electrode and a semi-transparent electrode, to form a microcavity. The disclosure describes a top-emitter structure using a patterned cathode to form a semi-transparent electrode over the gamut-defining color sub-pixels and a transparent electrode over the within-gamut sub-pixel. However, these structures, as described, do not meet the need for improved conductivity within the top, transparent electrode.
There is a need, therefore, for an improved method and structure for providing improved manufacturing yields and power distribution in an upper electrode, and a method of making thereof.